The present invention relates to a buffer storage control system, and more particularly, to a buffer storage control system having a priority circuit for requesting access of a buffer storage provided with such a circuit to prohibit acceptance of requests other than a by-pass request and a move-in request.
In general, a buffer storage control system mainly includes a central processing unit, a main storage, a memory control unit, a buffer storage, an instruction processing unit, and an arithmetic unit. When a data fetch request from, for example, an arithmetic unit, is accepted, the buffer storage checks whether or not the desired data exists in the buffer storage itself. If the data exists, the desired data is sent to the arithmetic unit, but if the desired data is not present, a request for one block of data, containing the required data, is sent to the main storage. When the requested data is delivered from the main storage unit through the memory control unit, the buffer storage delivers the requested data to the arithmetic unit, and in consequence, the relevant data and the next data, that is, one block of data containing by-pass data, are stored into the buffer storage itself.
The term by-pass operation means that in a block transfer, directly necessary data is not written in a buffer storage, but transferred to an instruction processing unit or an arithmetic unit, and the term move-in operation means that data for a block transfer is written in a buffer storage.
In the prior art, when the buffer storage has finished the by-pass operation, it can accept a next access request. Therefore, if the first move-in operation is not finished, when the next access address is requested in case the by-pass action and the next access address continues after another in succession, a missing-hit data or a Line Missing Detect (LMD) data in the next access is often produced to enter into a recycle sequence (Refer to FIG. 3). The term recycle sequence means that one action cannot be finished due to some cause and must be done over again.